If you've got any pics of him that would help us ID him and hopefully determine a course of action.
The BASICS:
Black Iguanas aka Spiny tails, (Most commonly seen are Ctenosaura similis or C. pectinata, but there are others found in the pet trade)
These guys have been finding their ways into the pet trade due to their unusual looks, and smaller size of 2-4ft.
Unfortunately they are not as easily adaptable as their green relatives, and many don't "tame" down. They tend to be much more flighty, and are VERY FAST. The fastest lizards, actually.
Here is an article on Spinies.
Central Pets.com (which basically is taken from the Anapsid.org site I listed below.)

There is, however, also a link to taxonomy, and if I'm not mistaken, takes you to a full taxonomy with pictures that might help you ID your ig.
Another to help you with ID:
Dealing with Iguanas in S. Florida
I believe my Spiny is a C. similis.
Some people claim all Spinies are strictly Herbivores. I found mine to be primarily Insectivorous at first, not even touching greens I offered. I just made sure to dust the crickets with a good calcium supplement, to ensure he was getting his proper intake. If I had roaches, I would have gone that route.
As much as some may dislike her, Melissa Kaplan had some of the only decent info I found that has worked for my Spiny Tail iguana.
Anapsid.org
In this article MK states most adult Spinies do well on a 95% herbivorous, 5% animal matter diet, and I don't disagree. In fact, their teeth are even different from the Green Iguana in that they are larger, sharper, and more serrated, perfect for a slightly omnivorous diet.
I was hoping mine would switch over to strictly veggies/greens/fruits, but it appears she is holding out on me.

In order to get mine to eat greens I had to place them INSIDE or just outside his hide. The first greens he took on his own (without me syringe feeding him) were a "Spring Mix" I purchased at Wal Mart (bagged, no less) and I dusted everything with calcium supplement since he really ate so little.
Cherry tomatoes (cut in 1/4s) thinly sliced apples, mango, pineapple, papaya, figs, cantaloupe.. pretty much anything it WILL eat, and is not considered harmful, I feed it. Very much a crash course for me with this lizard.
Intestinal parasites are common in those found in the pet trade. The problem comes in treating them when they aren't eating well to begin with, so they are so dehydrated its hard to get them healthy enough to treat.
I would advise soaks at least 2-3 times a week (sight unseen, more if he's really dehydrated) Misting or having a drip system would be advised as they don't usually drink from their water bowls. I have caught mine soaking ONCE, and he looked really confused as to what he was doing in the water..LOL
VenomMike was instrumental in helping me with my Spiny. I also am waiting on a member of another site to contact a friend that has had a Spiny for several years and put me in touch with him.
Get some pics of him, and hopefully I can help you or we can find someone who knows more about them
